1. Field of the Invention
The present invention relates to a process and an apparatus for producing a photosensitive resin printing plate for letterpress printing such as corrugated board printing, film printing, and seal and label printing. Particularly, the invention pertains to such a process and apparatus designed to improve the thickness accuracy of photosensitive resin printing plates.
2. Description of the Related Art
Generally, for producing a printing plate using a liquid photosensitive resin, as illustrated in FIG. 1 of the accompanying drawings, first a negative film 2 and a cover film 3 are attached tightly on a lower transparent substrate 1 by utilizing vacuum or other means, then a photosensitive resin layer 4' is laminated thereon by a suitable means such as a bottom-open bucket as shown in FIG. 2, and then a base film 7 and a masking film 8 are laid thereon as shown in FIG. 3. Thereafter, active light is applied via an upper transparent substrate 11 placed on the spacers 10 set for determining the thickness of the photosensitive resin printing plate to effect masking exposure for forming the base of the relief portion, after which active light is further applied via the negative film 2 from the lower transparent substrate 1 side to effect relief exposure for forming the image of the relief portion. Then the masking film 8 is removed and back exposure is conducted via the upper transparent substrate 11 for fixing the formed relief portion on the base film 7 in a stabilized way.
In some cases, final back exposure can be omitted.
Then the uncured portion is washed away with a suitable detergent to develop the cured portion, followed by post-exposure and drying to obtain a printing plate.
The following method is also prevalently used: A negative film 2 and a cover film 3 are placed tightly on a lower transparent substrate 1 without using a masking film, then a photosensitive resin layer 4' is laminated thereon, and a base film 7 is placed on this resin layer. Thereafter, active light is applied via an upper transparent substrate 11 placed on the spacers 10 set for determining the thickness of the printing plate to be produced, thereby to effect back exposure for forming the basal part of the printing plate, and then active light is further applied via the negative film 2 from the lower transparent substrate 1 side to effect relief exposure, followed by development, post-exposure and drying.
However, when a thick (usually 7 mm) and large-sized (over 1 m long) plate such as generally used for corrugated board printing or the like is made using the above techniques, the laminated resin thickness distribution tends to become nonuniform due to a decrease in the amount of the resin in the bucket with the advance of lamination, thereby resulting in an abnormal reduction of the plate thickness at the rear end portion of the laminate, which can cause improper printing. This phenomenon is attributable to the fact that the resin is laminated at a constant rate despite variation in the amount of liquid photosensitive resin amount in the bucket.
In order to overcome this problem, it has been proposed to reduce the resin laminating speed halfway in the process. This method, however, was not necessarily satisfactory, since as it was not suited for making plates of greater sizes and the required laminating speed reduction would affect the thickness accuracy of the plate produced.
Accordingly, the present invention is designed to eliminate these prior art problems and to provide a process for producing a thick and large-sized printing plate with high thickness accuracy in a stabilized way.
For attaining the same object as that of the present invention, a method has been proposed in which resin forming is carried out by making use of a combination of a multimanifold and a constant-rate resin feed pump (U.S. Pat. No. 4,383,759). This method, however, requires adjustment of several mechanical arrangements or an operation for altering the setting every time the thickness of the plate to be produced is changed in manufacturing the printing plates of different thicknesses. Such an operation is troublesome and subject or succeptable to mistakes. Further, there are required two types of metering pump according to the thickness of the plate to be produced (thin plate or thick plate), which fact complicates the process and makes the apparatus costly.
In contrast, the present invention features a compact combination of a speed controlling motor and control software, hence is low in cost, and allows for a very easy alteration of plate thickness with a simple plate thickness input operation alone, without requiring any other operations, by incorporating the device of U.S. Pat. No. 4,773,841.